Chaotic and non-chaotic response to quasiperiodic forcing: limits to predictability of ice ages paced by Milankovitch forcing

TL;DR

This paper investigates how quasiperiodic Milankovitch-like forcing can induce chaotic responses in models of ice ages, highlighting the limits of predictability in glacial cycle timings.

Contribution

It provides a quantitative comparison of nonlinear ice age models' responses to quasiperiodic forcing, revealing potential chaos and limited predictability.

Findings

Many models exhibit chaos under quasiperiodic forcing.

Simplest models may not show chaos, indicating model complexity matters.

Pacing of ice ages to forcing may have limited determinism.

Abstract

It is well known that periodic forcing of a nonlinear system, even of a two-dimensional autonomous system, can produce chaotic responses with sensitive dependence on initial conditions if the forcing induces sufficient stretching and folding of the phase space. Quasiperiodic forcing can similarly produce chaotic responses, where the transition to chaos on changing a parameter can bring the system into regions of strange non-chaotic behaviour. Although it is generally acknowledged that the timings of Pleistocene ice ages are at least partly due to Milankovitch forcing (which may be approximated as quasiperiodic, with energy concentrated near a small number of frequencies), the precise details of what can be inferred about the timings of glaciations and deglaciations from the forcing is still unclear. In this paper, we perform a quantitative comparison of the response of several low-order nonlinear conceptual models for these ice ages to various types of quasiperiodic forcing. By computing largest Lyapunov exponents and mean periods, we demonstrate that many models can have a chaotic response to quasiperiodic forcing for a range of forcing amplitudes, even though some of the simplest conceptual models do not. These results suggest that pacing of ice ages to forcing may have only limited determinism.